In electrophotographic devices such as copying machines and laser beam printers, since a toner is melted by heat and then fixed on a sheet, fixing is performed on a fixing part by applying high heat of around 200° C. Thereafter, the sheet is ejected by a paper injection roller and a paper injection guide (a paper injection rib). However, in accordance with the speeding-up in recent years, sheets and fixed toners are passed through paper injection parts while still being in a high temperature state. Therefore, toners are often brought into contact with paper injection rollers and the like in a semi-molten state in many cases, and heat-resistance and non-tackiness to toners have been required for paper injection rollers.
Fluorine resin products, which are excellent in non-tackiness, have been conventionally used for guide parts used for these paper injection rollers, paper injection guides and the like. It is known that such fluorine resin products are produced by injection molding by using, for example, a molten fluorine resin having a crystalline melting point of 250° C. or more and a viscosity at a melting point of 280 to 380° C. in the range of 1×103 to 1×106 poise (see Patent Documents 1 and 2).
The above-mentioned molten fluorine resin has a large tensile elongation at break. Therefore, even after molding and cooling to around ordinary temperature, when the die is subjected to die opening so that an injection molded body is separated from a gate, the resin in the gate and the resin that forms the molded body are separated from each other in the state that they have not been completely cut, and thus “stringing phenomenon” in which the resin is stretched like a string.
In a known injection molding method, in order to prevent such “stringing phenomenon”, when a resin molded body with a hole is produced by injection molding, in-mold processing is performed by providing a core pin so that the core pin can be freely moved forward and backward in the axis direction in a cavity and providing a gate part on the extension of the center axis of the core pin, so that a gate is occluded by the tip of the core pin to thereby completely cut the resin in the gate and the molded body during the formation of a through hole on the molded body by means of the driven core pin (see Patent Document 3). In addition, as an injection molding die for a rolling element for transporting sheets in an image forming device, an injection molding die to which a pinpoint gate is adopted is known (see Patent Document 4).